1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine which integrally incorporates a spark plug and an ignition coil and also incorporates a pressure detecting element for detecting a pressure of a combustion chamber.
2. Description of the Background Art
There are various kinds of conventional ignition devices for the internal combustion engine which integrally incorporates a spark plug and an ignition coil (refer to Japanese Patent Application Laid-open No. 2000-252040 and European Patent Application Laid-open No. 0907019).
Furthermore, Japanese Patent Application Laid-open No. 2000-277232 proposes an ignition device for an internal combustion engine which incorporates a pressure sensing element for detecting a pressure of a combustion chamber. According to this conventional ignition device, the pressure sensing element is disposed outside a cylindrical body of the spark plug. The signal lines of the pressure sensing element are taken out of the casing while they pass aside the ignition coil.
However, according to the ignition device for an internal combustion engine described in Japanese Patent Application Laid-open No. 2000-277232, it is necessary to secure a space between the ignition coil and the casing for taking the signal lines of the pressure sensing element out of the casing. Accordingly, the diameter of the casing necessarily increases. Furthermore, the signal lines of the pressure sensing element pass aside the ignition coil which generates noises. Therefore, these lines definitely require an effective countermeasure against noises.
In view of the foregoing problems, the present invention has an object to provide an ignition device for an internal combustion engine which integrally incorporates a spark plug, an ignition coil, and a pressure sensing element and installed in a cylinder head, according to which the increase in diameter of a casing can be suppressed and the output signal of the pressure sensing element is not adversely influenced by discharge noises.
To accomplish the above and other related objects, the present invention provides an ignition device for an internal combustion engine, including a spark plug having a center electrode accommodated in a cylindrical insulator for generating a spark discharge in a combustion chamber of an internal combustion engine, an ignition coil having a primary winding and a secondary winding for supplying high voltage to said spark plug, and a pressure sensing element for detecting a pressure of the combustion chamber. The spark plug, the ignition coil, and the pressure sensing element are accommodated in a cylindrical casing and installed in a cylinder head of the internal combustion engine. The insulator includes a plug side cylindrical portion having an inner space for accommodating the center electrode and a coil side cylindrical portion extending from the plug side cylindrical portion in a direction departing from the combustion chamber. The coil side cylindrical portion has one end protruding in the direction departing from the combustion chamber with respect to the primary winding and the secondary winding. And, the pressure sensing element is disposed next to the one end of the coil side cylindrical portion.
The pressure sensing element is disposed at a far side compared with the ignition coil with respect to the combustion chamber. The signal lines of the pressure sensing element can be taken out of the casing without passing aside the ignition coil. Thus, the diameter of the casing does not increase. The output signal of the pressure sensing element is not influenced by discharge noises generated from the ignition coil. There is no necessity of employing complicated layout for the signal lines.
It is preferably that a holding member for holding the pressure sensing element is disposed next to the one end of the coil side cylindrical portion via the pressure sensing element, an outer cylindrical surface of the insulator is brought into contact with a receiving surface formed on an inner cylindrical surface of the casing so that the insulator is placed in a predetermined position with respect to the casing, and the pressure sensing element and the insulator are pressed to the receiving surface of the casing by the holding member assembled with the casing.
A preload is always applied on the pressure sensing element. This assures sensor output accuracy against pressure variation in the combustion chamber. Furthermore, pressing the insulator to the receiving surface of the casing makes it possible to provide a hermetical contact between the receiving surface of the casing and the abutting surface of the insulator for preventing the combustion gas from leaking out via the clearance between the casing and the insulator.